Methods for determining ions in solution include flame photometry, atomic absorption photometry and ion-specific electrodes. Test strip formats have been disclosed in copending U.S. patent application Nos. 493,969; 493,982; 493,983; and 583,127 assigned commonly herein. The use of certain compounds and compositions which selectively isolate ions from a sample solution has become popular in ion-specific electrodes. These compounds, known as ionophores, have the capability of transporting ions into an electrode membrane causing a difference in potential which can be measured. Ion assays utilizing the ion/ionophore phenomenon include membrane electrodes, liquid/liquid partitioning, fluorescence and test strips.